1. Field of the Invention
The present invention relates to a circuit arrangement for supplying a controlled substantially constant level of electrical power to a discharge lamp in a load circuit, the arrangement including a feedback loop responsive to a comparison or difference between a power measure signal substantially corresponding to electrical power consumed by the lamp and a reference signal corresponding to a desired consumed power, which power measure signal is formed by a power measure signal forming circuit which includes a multiplier which receives at a pair of inputs respective lamp voltage and lamp current measure signals formed by sensing the load circuit.
2. Description of the Related Art
Such a circuit arrangement is known from U.S. Pat. No. 5,075,602, which is assigned to an affiliate of the assignee of the present application.
In the known circuit arrangement, the instantaneous value of current through a load circuit which includes a discharge lamp and the instantaneous value of voltage across the ends of the load circuit forming a resonant tank are multiplied in a four quadrant multiplier to form a product which is averaged to represent average power supplied to the load circuit. The four quadrant multiplier is more costly and complex than a single quadrant multiplier. Further, the power measured in this circuit arrangement is not only the power consumed by the discharge lamp, since it also includes losses of the resonant tank circuit and power consumed in filament heating.
In U.S. Pat. No. 5,734,232, which is also assigned to the affiliate of the assignee of the present application, the power measure signal is obtained by multiplying in a one quadrant multiplier the absolute values of instantaneous signals indicative of lamp voltage and current. In the event that capacitive current is present in the load circuit i.e. a phase shift exists between the lamp voltage and current waveforms, calculation in the manner indicated can lead to a substantial error by including reactive as well as real components.